added README.txt for postgres db

git-svn-id: https://svn.wso2.org/repos/wso2/trunk/commons@182528 a5903396-d722-0410-b921-86c7d4935375
updated file

git-svn-id: https://svn.wso2.org/repos/wso2/trunk/commons@182530 a5903396-d722-0410-b921-86c7d4935375
updated

git-svn-id: https://svn.wso2.org/repos/wso2/trunk/commons@182531 a5903396-d722-0410-b921-86c7d4935375